Voltage Drop Calculator
Korean Electrical Code formula e = (k × L × I) / (1000 × A) with auto distance-based limit comparison (3% / 5% / 6% / 7%).
Result
Enter circuit information and click Calculate.
Voltage Drop Limits & Wire Size by Distance
The most-searched values in one place. Left: the regulatory limit. Right: which cross-section actually clears it.
Allowed Voltage Drop by Distance (Korean wiring code)
| Run length | Allowed drop |
|---|---|
| up to 60m | 3% |
| up to 120m | 5% |
| up to 200m | 6% |
| over 200m | 7% |
Drop by Distance × Cross-section (220V single-phase 2-wire · 20A · copper)
Each cell is the voltage drop percentage. Red marks combinations that exceed the limit for that distance.
| Distance | 2.5sq | 4sq | 6sq | 10sq | 16sq | 25sq |
|---|---|---|---|---|---|---|
| 10m | 1.3% | 0.8% | 0.5% | 0.3% | 0.2% | 0.1% |
| 20m | 2.6% | 1.6% | 1.1% | 0.6% | 0.4% | 0.3% |
| 30m | 3.9% | 2.4% | 1.6% | 1.0% | 0.6% | 0.4% |
| 50m | 6.5% | 4.0% | 2.7% | 1.6% | 1.0% | 0.6% |
| 80m | 10.4% | 6.5% | 4.3% | 2.6% | 1.6% | 1.0% |
| 100m | 12.9% | 8.1% | 5.4% | 3.2% | 2.0% | 1.3% |
| 150m | 19.4% | 12.1% | 8.1% | 4.9% | 3.0% | 1.9% |
| 200m | 25.9% | 16.2% | 10.8% | 6.5% | 4.0% | 2.6% |
The matrix assumes the fixed conditions above. Different voltage, current, phase type or conductor material changes the result — use the calculator for actual design. Aluminum drops about 1.6× more under the same conditions.
Basis: Korean Internal Wiring Code 1410-1 (drop limit by distance) · KS C IEC 60364
What this tool does
The Voltage Drop Calculator takes conductor cross-section, current, run length, and phase type, then computes the voltage drop (V) and drop percentage (%) and compares it against the distance-based allowable limits in Korea's KEC wiring standard (formerly the Naeseon code). It supports both copper and aluminum conductors and applies the correct coefficient for single-phase 2-wire, single-phase 3-wire, 3-phase 3-wire, and 3-phase 4-wire circuits. Use it to confirm whether a wire gauge is adequate for the run distance and whether the voltage at the far end stays within equipment tolerance.
Who uses this
- Check whether the drop exceeds the limit when a load is far from the distribution panel
- Decide whether to step up to a larger wire gauge because of excessive drop
- Compare voltage drop between single-phase and 3-phase wiring options
- Assess the extra drop when using aluminum conductors instead of copper
- Learn the voltage-drop formula for Korean electrician licensing exams
How to use (4 steps)
- 1Enter the line (or phase) voltage (V) and load current (A) — e.g., 220V single-phase, 380V 3-phase.
- 2Enter the run length (m, one-way) — the actual wire length from the panel to the load.
- 3Select the conductor cross-section (㎟) from the KS standard sizes, the conductor material (copper/aluminum), and the phase type (single-2, single-3, 3-phase-3, 3-phase-4).
- 4Read the voltage drop (V), drop percentage (%), the distance-based limit, and the pass/fail verdict. If it fails, increase the cross-section or shorten the run.
Formula & basis (KEC / former Naeseon code)
■ Voltage drop e = (coefficient × L × I) ÷ (1000 × A) L = run length (m, one-way), I = current (A), A = cross-section (㎟) ■ Coefficient by phase type (copper) Single-phase 2-wire 35.6 3-phase 3-wire 30.8 Single-phase 3-wire / 3-phase 4-wire 17.8 Aluminum conductor: coefficient × 1.6 ■ Drop percentage e(%) = (e ÷ V) × 100 ■ Allowable drop by distance (combined feeder + branch) ≤ 60m — 3% / ≤ 120m — 5% / ≤ 200m — 6% / > 200m — 7%
Worked examples
Example 1: 220V single-phase, 20A, 50m, 4㎟ copper
e = (35.6 × 50 × 20) ÷ (1000 × 4) = 8.9V. Drop = 8.9 ÷ 220 × 100 ≈ 4.05%. The ≤60m limit is 3%, so this fails. Increasing to 6㎟ or larger brings it down to about 2.7% and passes.
Example 2: 380V 3-phase 3-wire, 30A, 80m, 10㎟ copper
e = (30.8 × 80 × 30) ÷ (1000 × 10) = 7.39V. Drop = 7.39 ÷ 380 × 100 ≈ 1.95%. Within the ≤120m limit of 5%, so it passes.
Example 3: switching from copper to aluminum
Same as Example 1 but aluminum: coefficient 35.6 × 1.6 = 56.96 → e ≈ 14.24V, drop ≈ 6.47%. Aluminum's lower conductivity raises the drop by about 60% at the same gauge — you must size up at least one step.
Frequently asked questions
Is the run length one-way or round-trip?
Enter the one-way length (panel to load). The return current path is already accounted for in the phase coefficients (35.6, 30.8, 17.8), so do not double the distance.
What are the 3% / 5% allowable limits based on?
They come from Korea's KEC wiring standard (formerly the Naeseon code): 3% for runs ≤60m, 5% for ≤120m, 6% for ≤200m, 7% over 200m. These are end-of-run values combining feeder and branch circuits, set to keep lighting and equipment within their normal operating voltage.
What should I do if the drop exceeds the limit?
Either (1) step up the conductor cross-section, (2) shorten the run by relocating the panel, or (3) switch single-phase to 3-phase (lower coefficient). Increasing the wire gauge is the most common fix.
Why does aluminum wire have a larger voltage drop?
Aluminum's conductivity is about 61% of copper's, so at the same cross-section its resistance — and voltage drop — is roughly 1.6× higher. This calculator applies a ×1.6 factor automatically when aluminum is selected. To match copper's drop, size aluminum one or two steps larger.
Does it account for DC or power factor?
This is a simplified AC calculation assuming a power factor of 1, ignoring line reactance (inductance). Long, high-current, or low-power-factor circuits are more affected by reactance — use a licensed engineer or design software for precise work.
Cautions
- •Simplified AC calculation assuming power factor 1 and ignoring reactance. Precise design needs full line-impedance calculation.
- •Enter the one-way length; the round-trip path is already in the phase coefficient.
- •Allowable limits follow the KEC distance bands; actual design depends on feeder/branch allocation and equipment characteristics.
- •Reference calculation only. Have an electrical safety manager or qualified engineer review actual installations.
- •Cross-section must be a KS standard size.
Last reviewed: 2026-06-17